1. Field of the Invention
The present invention concerns a method to expand an analog signal and, more particularly, to expand at least one part of an analog signal which is substantially rectangular or square shaped. It also concerns a device to implement this method.
2. Description of the Prior Art
In certain manufacturing methods, it is useful to be able to expand or enlarge at least one part of an analog signal in order to use this part of the signal as a control element in the manufacturing process. Thus, in semiconductor technology, during the etching process, photomultipliers are used to convert a light intensity into an analog electrical signal which is then processed by a computer. Now, the valuable part of the analog signal given out by the photomultiplier has an amplitude of some millivolts. This part of the signal must therefore be levelled so that it can subsequently be used.
The presently used device consists of an operational amplifier, the gain and zero shift of which are achieved by means of two potentiometers. This is therefore a manual system which needs to be set at each process and product change.
An object of the invention, therefore, is to propose a method for the expansion of at least one part of an analog signal which is substantially rectangular or square shaped, which automatically levels the analog signal.
Another object of the present invention is to propose a device to implement the expansion method of the present invention. This device has the advantage of automatically checking the gain and of automatically shifting the zero of the input analog signal.
Consequently, the present invention relates to a method for the expansion of at least one part of an input analog signal which is substantially rectangular or square shaped, said method consisting in multiplying the input signal by a programming signal which is a function of the rising edge of the analog signal and of a maximum threshold, and then subtracting a subtraction signal, which is a function of a top threshold and a bottom threshold.
According to another feature of the present invention, the programming signal is computed for each analog signal while the subtraction signal is computed during the emission of a resetting signal and then memorized and used for the following analog signal.
According to a preferred embodiment, the programming signal consists of a slope signal which starts at the detection of the rising edge of the analog signal and stops when the output signal corresponds to the maximum threshold, the slope gradient depending on a clock signal, and the subtraction signal consists of a slope signal which starts when the output signal corresponds to a top threshold and stops when the output signal corresponds to a bottom threshold, the slope gradient depending on a clock signal.
The present invention also relates to a device to implement the above method, comprising an amplifier that receives the input signal, the gain of which is controlled by a programming signal, means to generate the programming signal as a function of the rising edge of the analog signal and of a maximum threshold, a subtractor receiving the output signal from the amplifier and a subtraction signal and means to generate the subtraction signal as a function of a top threshold and a bottom threshold.
According to a preferred embodiment of the device, the means to generate the programming signal comprise a slope generator giving the programming signal; a first control logic circuit controlling the various operations of resetting, starting, stopping the slope generator and memorizing the signal; a derivative computing circuit receiving the analog signal as an input and applying a starting pulse to the control logic circuit; and a first comparator comparing the output analog signal with a maximum threshold and sending to the control logic circuit a stop signal.
The means to generate the subtraction signal comprise a slope generator delivering the subtraction signal; a second control logic circuit controlling the different operations for resetting, starting, stopping the slope generator and memorizing the signal; a second comparator comparing the output analog signal with a top threshold and sending the starting pulse to the control logic circuit; and a third comparator the output analog signal with a bottom threshold and sending a stop signal to the control logic circuit.
Furthermore, the second control logic circuit further has a reset circuit starting up the means for generating the subtraction signal signal and means to memorize the subtraction signal obtained until a new resetting operation of the reset circuit is performed.
Preferably, the slope generators consist of a digital counter connected to a digital/analog converter and a clock circuit determining the slope gradient.
According to another feature of the present invention, the device further has means for preamplifying and shaping the input analog signal.